The removal of minute contaminants from a surface of a substrate or wafer continues to be an important operation in the manufacture of semiconductor devices, photomasks, and the like. Most minute contaminants tenaciously hold to a surface and require a relatively large force to remove them. The use of ultrasonic energy to enhance the cleaning action of solutions used on semiconductor substrates and wafers is now well established (see, e.g., U.S. Pat. No. 4,854,337). Ultrasonic agitation has also been used to enhance the action of etching solutions on semiconductor substrates and wafers. Such cleaning and etching processes conventionally use transducers operating at intermediate frequencies, i.e., 20-50 KHz., and, more recently, high frequencies of 0.2 to 5.0 MHz. The use of high frequency, or megasonic, agitation of solutions has resulted in improved cleaning and etching, particularly on substrates and wafers with very small, micron size components or elements disposed thereon. Further, use of high frequency agitation provides a gentler cleaning action on the wafers and substrates than that obtainable with an intermediate frequency agitation. As a result, damage to micro sized components during cleaning/etching operations is significantly reduced.
In a conventional implementation, one or more megasonic transducers are mounted in association with a bottom surface of the cleaning tank. Such structures have the inherent drawback that transducer placement interferes with laminar flow of the chemical within the recirculation tank. This results in stagnant zones within the tank with no flow of chemical. Such zones are undesirable because they tend to be highly contaminated with particulates which can redeposit on the wafers as the wafers are being drawn through these areas, either on immersion into or withdrawal from the recirculation tank. High particulate content within stagnant zones can also contaminate fresh chemical if the flow patterns are disturbed upon wafer immersion.
Various attempts have been made to provide mechanical agitation to tank type cleaning and etching baths employing megasonic frequency generating transducers. As described in U.S. Pat. No. 4,854,337, such arrangements typically comprise apparatus to move the substrate/wafer containing cassettes through an energy beam created by the transducers, thereby ensuring that all portions of the wafer surfaces are treated by the energy beam. Unfortunately, creating the necessary agitation can be quite expensive and typically results in a much larger apparatus than otherwise necessary.
Thus, an improved technique wherein megasonic cleaning/etching of devices occurs within a laminar flow tank is needed and will achieve widespread acceptance in the semiconductor manufacturing industry.